The reaction layer of a fuel cell is formed between a proton exchange membrane and a backing layer and supports a catalyst for accelerating an electrochemical reaction. A combination of the reaction layer and the catalyst layer constitutes the electrodes of the fuel cell. In the reaction layer on an air cathode side, for example, protons passing through the proton exchange membrane and electrons transferred to the air cathode are conducted up to the catalyst, thus binding oxygen and protons diffused onto the catalyst. Namely, the reaction layer needs to exhibit both proton conduction and electronic conduction in order to improve the transfer loss of oxygen, protons and electrons. To this end, a mixture of poly electrolyte having catalysts supported on surfaces such as carbon particles (exhibiting electronic conduction) and Nafion (trade name, manufactured by E.I du Pont de Nemours, this applies hereafter) which exhibits ionic conduction is used in the fuel cell.
However, if a material having ionic conduction and a material having electronic conduction are used together, it is difficult to mix them up completely uniformly. As a result, protons and electrons cannot be uniformly transferred to all catalyst particles.
To solve this disadvantage, there has been proposed a mixed conductor which exhibits both ionic conduction and electronic conduction using one material.
For example, organic mixed conductors are disclosed in JP2001-202971A, JP2001-110428A, JP2003-68321A and JP2002-536787A.
In addition, inorganic mixed conductors conducting electrons and oxygen ions are disclosed in JP1998-255832A, JP1999-335165A, JP2000-251533A, and JP2000-18811A.